Control circuits have been used for many years in controlling the actuation of valves. Particular circuitry has been used in viscosity measuring devices, for example, particularly of the moving piston type wherein a piston moves, either by freely falling, or by being driven, from a first or beginning position within a liquid whose viscosity is to be measured, to a second or end position, the time of such movement between the beginning and end positions being proportional to the viscosity of the liquid.
Such devices can be used, for example, to monitor in a generally continual manner the viscosity of a liquid so as to maintain such viscosity within acceptable limits. In such monitoring context, control circuitry is used to actuate an air valve, a liquid flow valve, or other means, for raising a piston therein to its known first position within a liquid sample. The piston is then permitted to fall freely, or is otherwise driven to move, within the liquid until it reaches a known second position, at which time it actuates a limit switch to permit a measurement of the time of such movement, the time then representing a measure of the viscosity. So long as the piston reaches its end position in a time period within an acceptable range, the viscosity is within the desired limits. The monitoring device continues to monitor the viscosity on a cyclical basis.
If, however, the viscosity becomes too high, the piston will not reach its end position within the desired time range and it is necessary to actuate a further valve for supplying solvent to the liquid so as to reduce its viscosity. On the other hand, if the viscosity is too low, the piston reaches its end position too quickly and it is necessary to actuate a valve for supplying a thickener to the liquid to increase its viscosity.
The control of the operation of the valves and the monitoring of the actuation of a limiter switch has up to this time required the use of relatively complex circuitry and electro-mechanical assemblies which ar relatively costly to fabricate.
It is desirable to perform the desired operations for controlling such viscosity measuring device in a simpler manner, using fewer components, and accordingly permitting such circuitry and assemblies to be fabricated at much less cost.